1. Field of the Invention
The present invention relates to a three-dimensional image capturing device by which a three-dimensional shape of a measurement subject, which is to be measured, is captured by a time-of-flight measurement.
2. Description of the Related Art
A three-dimensional measurement using a three-dimensional image capturing device is classified as an active system, in which light, an electric wave or sound is radiated onto a measurement subject, and a passive system in which the light, electric wave or sound is not output. The active system comprises the time-of-flight measurement, a phase detection using a modulated light wave, a triangulation, amoirxc3xa9 topography, and soon, and the passive system comprises a stereo vision system, and so on.
An active system device is very bulky in comparison with that of the passive system, since the device requires a laser beam output mechanism. However, the active system device is superior regarding a distance measurement resolution, a measuring time, a measuring range and so on, and thus, despite the bulkiness, the device is utilized in various fields. In a three-dimensional image capturing device, described in xe2x80x9cMeasurement Science and Technologyxe2x80x9d (S. Christies et al., vol.6, p.1301-1308, 1995), a pulse-modulation laser beam irradiates a measurement subject, and a reflected light beam, which is reflected by the measurement subject, is received by a two-dimensional CCD sensor to which an image intensifier is attached, so that an image signal, corresponding to the reflected light beam, is converted to an electric signal. ON-OFF control of the image intensifier is carried out by a gate pulse, which is synchronized with the pulse radiation of the laser beam. According to the device, since an amount of received light, based on the reflected light beam from the measurement subject, which is positioned far from the device, is less than that of received light based on a reflected light beam from a measurement subject, which is close to the measurement subject, an output corresponding to a distance between the measurement subject and the device can be obtained for each pixel of the CCD.
In a conventional three-dimensional image capturing device as described above, if the device is constructed in such a manner that information, which is used for correcting a reflectance to improve the accuracy of the distance measurement, is sensed, it may be necessary to expand the output range of the imaging device, such as CCD. However, merely expanding the output range of the imaging device will cause further difficulties in sufficiently improving the accuracy of the distance measurement.
Therefore, an object of the present invention is to improve the accuracy of the distance measurement when sensing a three-dimensional shape of a measurement subject without substantially expanding the output range of the imaging device.
According to the present invention, there is provided a three-dimensional image capturing device, comprising a light source, a first reflected light beam component sensing processor, a second reflected light beam component sensing processor and a distance calculation processor.
The light source irradiates a distance measuring light beam to a measurement subject. The distance measuring light beam is a pulsed beam. The measurement subject reflects the distance measuring light beam to generate a reflected light beam pulse. The first reflected light beam component sensing processor senses the reflected light beam pulse for a first constant period, which starts at a fall of the distance measuring light beam, to sense a first reflected light beam component including a fall of the reflected light beam pulse. The second reflected light beam component sensing processor senses the reflected light beam pulse for a second constant period, which has the same length as the first constant period and starts earlier than the fall of the distance measuring light beam, to sense a second reflected light beam component which includes the fall of the reflected light beam pulse. The distance calculation processor obtains the distance from the device to each point of the surface of the measurement subject based on the first and second reflected light beam components.